Known coating-type magnetic recording media (magnetic tapes including video tapes and the like) include multi-layer structure types consisting of a magnetic layer composed of magnetic powder dispersed in resin, a base film, and a nonmagnetic layer composed of nonmagnetic powder dispersed in a resin that is located between the magnetic layer and the base film as an underlayer. Use of such a multi-layer structure improves the electromagnetic conversion characteristics of the magnetic recording medium to enable high recording density. The reliability of the tape is also enhanced owing to improved durability. In this specification, the terms “powder for underlayer” and “underlayer powder” are used to mean a powder for forming the nonmagnetic layer (underlayer) of such a multi-layer structure coating-type magnetic recording medium.
The incessant rise in information volume in recent times has increased the need for recording media with higher recording capacity and reliability. This in turn has produced a need to improve the different properties of the underlayer powder used to fabricate multi-layer structure coating-type magnetic recording media. The basic properties required of an underlayer powder include ability to form an underlayer with high surface smoothness when applied to the base film and the ability to produce an underlayer composed of a film coating having high strength.
An underlayer having a smooth surface enhances the smoothness of the overlaid magnetic layer, making it possible to obtain a magnetic recording medium having excellent electromagnetic conversion characteristics that enable high recording density. Strength enhancement of the underlayer is desirable because the thinner tape thicknesses necessitated by rising tape capacity makes it important to achieve high recording density and for this to ensure dimensional stability. A stronger underlayer helps to upgrade reliability by boosting the strength of the tape proper.
In order to satisfy these requirements, an underlayer nonmagnetic powder generally must have excellent dispersibility at the paint-making stage, high particle strength, and high adherence to the resin. When a coating film is applied using a nonmagnetic powder with these properties, a smooth, high-strength underlayer can be formed.
The main nonmagnetic powders conventionally used for underlayer formation are spherical titanium oxide powder and acicular iron oxide powder. Multi-layer structure magnetic recording media having an underlayer are taught by, for example, JP Sho-63-187418A and Hei-4-167225A.
In addition, Hei-6-60362A, Hei-6-139553A, Hei-6-215360A, Hei-7-192248A and Hei-9-170003A, for example, indicate characteristic values when acicular ion oxide and the like are used as nonmagnetic powder for underlayer formation in such multi-layer structure magnetic recording media.